It is known that a cross-sectional shape of a boundary of weld metal (weld portion) in a weld portion of steel material (hereinafter, a weld-boundary cross-sectional shape) influences the strength of the steel material. Patent Literature 1, for example, discloses that the weld-boundary cross-sectional shape of a weld portion of a steel pipe influences the toughness of the weld portion. Consequently, non-destructive evaluation technologies by imaging the weld-boundary cross-sectional shape have been developed in terms of quality control and quality assurance of the steel material. Patent Literature 2, for example, discloses an imaging technology by receiving reflected waves from a boundary between a base metal portion and a weld portion. Specifically, this technology makes an image by receiving reflected waves of ultrasonic waves transmitted at an angle toward the boundary between the base metal portion and the weld portion and identifying reflection points from the received reflected waves while moving a probe or switching vibrators (elements) of an array probe. Furthermore, Patent Literature 3 discloses a tandem measurement technology for detecting ultrasonic waves by separating a receiving device and a transmitting device of the ultrasonic waves.
In general, the orientation of crystal structure is aligned in a weld portion, and thus the weld portion has acoustic anisotropy. According to Patent Literature 1, for the steel material for which the acoustic anisotropy of weld portion is large such as austenitic stainless steel, the difference between acoustic impedance (=medium density×sound velocity) of base metal portion Z1 and acoustic impedance of weld portion Z2 is of a relatively large value. For example, when sound velocity of a base metal portion V1 is 3200 m/s and sound velocity of a weld portion V2 is 2500 m/s, because the medium density is a substantially constant value, the ratio of the acoustic impedance of the base metal portion Z1 and the acoustic impedance of the weld portion Z2 is 1 to 0.78.
Now, it is known that the reflectivity per unit area at an interface between different media depends on the acoustic impedance of both. According to Non-Patent Literature 1, the reflectivity (sound pressure) per unit area rab at the interface when the ultrasonic waves are incident perpendicularly from the medium of acoustic impedance Za to the medium of acoustic impedance Zb can be expressed as the following Expression 1.
                              r          ab                =                                            Z              b                        -                          Z              a                                                          Z              b                        +                          Z              a                                                          (        1        )            
Thus, for the steel material in the foregoing example for which the acoustic anisotropy of the weld portion is large, the reflectivity per unit area r12 at the interface when the ultrasonic waves are incident perpendicularly from the base metal portion of acoustic impedance Z1 to the weld portion of acoustic impedance Z2 is −0.12 according to Expression 1. The reflectivity in a negative value here represents that the phase of ultrasonic waves is inverted, and thus the reflectivity per unit area is 12%.